Generally, the process for manufacturing integrated circuits on a silicon wafer substrate typically involves deposition of a thin dielectric or conductive film on the wafer using oxidation or any of a variety of chemical vapor deposition processes; formation of a circuit pattern on a layer of photoresist material by photolithography; placing a photoresist mask layer corresponding to the circuit pattern on the wafer; etching of the circuit pattern in the conductive layer on the wafer; and stripping of the photoresist mask layer from the wafer. Each of these steps, particularly the photoresist stripping step, provides abundant opportunity for organic, metal and other potential circuit-contaminating particles to accumulate on the wafer surface.
In the semiconductor fabrication industry, minimization of particle contamination on semiconductor waters increases in importance as the integrated circuit devices on the wafers decrease in size. With the reduced size of the devices, a contaminant having a particular size occupies a relatively larger percentage of the available space for circuit elements on the wafer as compared to wafers containing the larger devices of the past. Moreover, the presence of particles in the integrated circuits compromises the functional integrity of the devices in the finished electronic product. Currently, mini-environment based IC manufacturing facilities are equipped to control airborne particles much smaller than 1.0 μm, as surface contamination continues to be of high priority to semiconductor manufacturers. To achieve an ultra-clean wafer surface, particles must be removed from the wafer, and particle-removing methods are therefore of utmost importance in the fabrication of semiconductors.
The most common system for cleaning semiconductor wafers during wafer processing includes a series of tanks which contain the necessary cleaning solutions and are positioned in a “wet bench” in a clean room. Batches of wafers are moved in sequence through the tanks, typically by operation of a computer-controlled automated apparatus. Currently, semiconductor manufacturers use wet cleaning processes which may use cleaning agents such as deionized water and/or surfactants. Other wafer-cleaning processes utilize solvents, dry cleaning using high-velocity gas jets, and a megasonic cleaning process, in which very high-frequency sound waves are used to dislodge particles from the wafer surface. Cleaning systems which use deionized (DI) water currently are widely used in the industry because the systems are effective in removing particles from the wafers and are relatively cost-efficient. Approximately 4.5 tons of water are used for the production of each 200-mm, 16-Mbit, DRAM wafer. In the final process tank, the water and other rinse fluid is removed from the wafer surface using a solvent such as isopropyl alcohol (IPA). IPA is an organic solvent known to reduce the surface tension of water.
In one type of IPA drying method, wet substrates are moved into a sealed vessel and placed in the processing region of the vessel. An IPA vapor cloud is generated in a vapor-generating region of the vessel and is directed into the processing region, where it removes water from the wafers. This drying technology is highly effective in removing liquid from the wafers, but is not easily adaptable to single vessel systems in which chemical processing, rinsing, and drying can be carried out in a single vessel.
Environmental concerns have given rise to efforts to improve drying technology in a manner that minimizes IPA usage. One such improved drying technology is the Marongoni technique. In one application of the Marongoni technique, an IPA vapor is condensed on top of the rinse water containing the wafers while the wafers are slowly lifted from the processing vessel. The concentration of the dissolved vapor is highest at the wafer surfaces and lower at the regions of the rinse fluid that are spaced from the wafer surfaces. Because surface tension decreases as IPA concentration increases, the surface tension of the water is lowest at the wafer surface where the IPA concentration is highest. The concentration gradient thus results in “Marongoni flow” of the rinse water away from the surfaces of the wafers. Rinse water is thereby stripped from the wafer surfaces, leaving the wafer surfaces dry.
One example of a conventional Marongoni system 10 which employs the Marongoni technique is illustrated schematically in FIG. 1. The system 10 includes an IPA container 12 which contains a supply of isopropyl alcohol. A valve 15 is provided between an IPA outlet line 14 which leads from the IPA container 12 and an IPA transfer line 16 which leads to a substrate cleaning tank 18. A nitrogen gas supply 20 is provided in fluid communication with the IPA transfer line 16 through a nitrogen flow line 22, which is typically fitted with a valve 24, a filter 26 and a flow meter 28. The nitrogen gas supply 20 is typically further provided in fluid communication with the IPA container 12 through a second nitrogen flow line 22a, typically fitted with a valve 24a, a filter 26a and a flow meter 28a. 
In a substrate drying process using the system 10, a substrate (not shown) is first rinsed in the substrate cleaning tank 18 using DI (deionized) water. Next, nitrogen gas is distributed from the nitrogen gas supply 20 to the IPA transfer line 16 through the nitrogen flow line 22, while IPA vapor is distributed from the IPA container 12 to the IPA transfer line 16 through the IPA outlet line 14 and valve 15. The IPA vapor mixes with the nitrogen gas in the IPA transfer line 16, and is introduced into the substrate cleaning tank 18. A meniscus-shaped gradient is formed along the interface between the surface of the substrate and the DI water, and as the substrate is removed from the substrate cleaning tank 18, the water flows along the meniscus portion and is thereby removed from the substrate, with no water remaining on the substrate upon complete removal of the substrate from the tank 18.
During the drying operation of the conventional Marongoni system 10 heretofore described, the IPA vapor is carried by nitrogen gas from the IPA container 12, through the IPA transfer line 16 and to the substrate cleaning tank 18 typically at atmospheric pressure. However, as it has a boiling point of 82° C. at atmospheric pressure, the IPA condenses into liquid from the vapor state upon cooling below this temperature. Accordingly, during transit through the IPA transfer line 16, the nitrogencarried, vaporized IPA tends to condense to form liquid IPA. Upon entry into the substrate cleaning tank 18, the liquified IPA tends to induce formation of water marks in and carry particles into deep sub-micron trenches formed in the substrate during the course of IC fabrication. This adversely affects the yield of devices on the substrate. Furthermore, the filter 26a tends to become blocked by the liquified IPA. Accordingly, a system is needed for maintaining the IPA in a vaporized state throughout drying of substrates after washing in order to prevent formation of water marks in the devices formed on the substrate.
An object of the present invention is to provide a new and improved system for drying substrates.
Another object of the present invention is to provide a substrate drying system in which control of drying fluid vapor concentration is enhanced, particularly for advanced process applications.
Another object of the present invention is to provide a new and improved system for drying substrates, which system prevents formation of water marks and deposition of particles on the substrates induced by prematurely-condensed or liquified drying fluid such as IPA (isopropyl alcohol).
Still another object of the present invention is to provide a system for maintaining a drying fluid in a vaporized state as the drying fluid is transported from a container to a substrate cleaning tank and during drying of substrates in the substrate cleaning tank.
Another object of the present invention is to provide a new and improved substrate drying system which may be adapted for use in Marongoni-type substrate drying systems for drying substrates.
Yet another object of the present invention is to provide a system for lowering the boiling point of a drying fluid in order to prevent liquification of the drying fluid during drying of substrates after washing.
A still further object of the present invention is to provide a system which reduces the boiling point of a drying fluid for substrates by reducing pressure in the system during drying of substrates in order to prevent liquification of the drying fluid and formation of water marks particularly in deep trenches formed on the substrates.
Yet another object of the present invention is to provide a new and improved system for enhancing wafer yield in the fabrication of semiconductor integrated circuits on substrates.
A still further object of the present invention is to provide a system which reduces the presence of particles remaining on substrates after washing and drying of the substrates.
Yet another object of the present invention is to provide a new and improved method for drying substrates after washing.